The present invention relates to a current sensing device. It relates more particularly to a device for measuring the difference between the currents flowing in two wires having a very large voltage difference therebetween, such as the collector and cathode circuits of a traveling wave tube, which is capable of accurately measuring small current differences over a wide range of ambient temperatures.
In a traveling wave tube (TWT), as is well known, a stream of electrons interacts with an electromagnetic wave carried on a helically wound conductor, which is generally referred to as the helix. The stream of electrons is released from a cathode and travels within the helix toward a collector. Those electrons reaching the collector constitute the collector current. According to the positive current convention, the collector current will be considered herein as flowing from the collector to the cathode.
Those electrons which do not reach the collector, but go astray and impact the helix, constitute the helix current. The sum of the helix current and the collector current therefore equals the cathode current.
The helix current is a measure of the quality or effectiveness of the operation of a TWT, and should be as low as possible. However, even in a very good TWT, a helix current of approximately 0.5 percent of the cathode current is present. On the other hand, if the helix current is 15-20 percent of the cathode current, this is considered marginal or inadequate performance for the TWT. Excessive helix current, moreover, must be prevented to avoid catastrophic hardware damage.
It is necessary, therefore, to continually monitor the helix current of the TWT to ensure immediate detection of unsafe or undesirably high helix current levels, say, above about 8 to 12 percent of the cathode current. However, helix current sensing is rendered difficult by the large potential differences encountered in the TWT. For example, in a typical miniature TWT the cathode electrode will operate at a voltage of approximately -4,000 volts and the collector at a potential of approximately +2,000 volts with respect to the cathode, that is, an absolute level of -2,000 volts. Moreover, because of radio frequency design considerations the helix itself is held at ground potential, that is, at approximately 4,000 volts above the cathode voltage.
The high voltages in the TWT prevent the employment of ordinary electronic techniques for measuring the helix current. In the special case where a system employs only a single TWT, the helix current can be measured directly, since it is the current that flows in the ground terminal of the power supply of the TWT. In many systems, however, a plurality of TWTs are used. For example, in phased array systems and in many airborne radars and similar devices, arrays of TWTs are used. Conventional techniques are not able to measure the individual helix current of each TWT.
Helix currents can be measured indirectly, however, by subtracting the measured collector current from the measured cathode current, since the current that flows at the cathode electrode equals the collector current plus the helix current.
To indirectly detect a helix current by this method, in a high voltage environment, prior art techniques have employed the inherent physical properties of a magnetic device having a core and at least two windings, which develops a magnetic flux in response to an applied current. The flux that is produced in such a magnetic device is related to the number of turns in its windings and the current through the windings. Such technique is particularly useful, since the windings of a magnetic device can easily be insulated to withstand the large voltage differences within the TWT power supply. A magnetic flux indicative of a helix current can be produced by employing two identical, electrically isolated windings with oppositely directed current flows. The resulting flux will be related to the difference between the two currents. To measure helix current, the cathode and collector currents of the TWT are applied to such a device, producing a flux which is proportional to the helix current. The flux, in turn, may be sensed by measuring the inductance of a sense winding on the device. This technique is based on the fact that the flux influences the permeability of the magnetic core of the device, and the permeability in turn determines, by a known function, the inductance of the sense windings.
However, in practice it is difficult to reliably relate a flux to an inductance by this method, since permeability varies according to several factors, including core material, flux density, and temperature. Typically, pemeability varies by a factor of 3:1 over a temperature range of -55.degree. C. to +125.degree. C., which is the temperature range over which TWTs must operate in many applications.
Therefore, although magnetic devices can be adapted to operate in the high-voltage environment of TWTs, their usefulness as helix current indicators over extended temperature ranges is severely limited.
Another limitation of prior art techniques is that they have principally employed an "incremental" approach, in which the magnetic device is operated in magnetic saturation for all currents except a narrow band of current levels in the vicinity of a predetermined desired helix current. Such predetermined helix current level will be referred to herein as a helix trip current, since it is the current limit above which an alarm is triggered to indicate an impending tube failure or other operational problem.
A magnetic device follows a magnetization curve which varies between negative and positive saturation levels in response to respective negative and positive currents that pass therethrough. The incremental approach uses the changing magnetic flux of the device as it passes between negative and positive saturation through the non-saturated region to produce an electrical pulse, which is processed by sensing circuitry to indicate that the trip helix current has been passed. In order to obtain a substantial pulse, such techniques employ a high rate of change of magnetic flux.
This method can employ windings having many turns to give very sensitive sensing of helix currents at or near the trip current point. However, the method is disadvantageous in that for a very rapid rise in helix current, which can easily occur in the TWT in normal operation, such as when an arc occurs or when a TWT becomes "gassy", the trip point may be passed so rapidly that the circuitry fails to respond. A sufficient degree of high-frequency response cannot easily be provided. Further, this method does not solve the problem of the large magnetic permeability variations over temperature noted above. Also, the incremental method cannot provide a reading of the actual helix current.